Do you get an reply saying that there are programs providing java/javac?

If you get one program you'll see the path to the respective executable.
If there is no "+" in front type the selection number "1", hit <Enter> and follow the instructions.
If you see several ones you have the option to choose which one to make the default, just like above.

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So you have two java versions installed, and 1.6 is set as the default (I assume that's what you desire).

Yes, JAVA_HOME should be "/usr/lib/jvm/jre-1.6.0-openjdk" in this case.

You could type "2" and hit <Enter> and follow the instructions to set 1.6 as the default again. Maybe this will set JAVA_HOME, but if it doesn't see my first comment where to add it.

Best use /etc/profile, so all users will have the correct version as the default.
You should also check your PATH variable. Does it contain $JAVA_HOME/bin (the resolved version, that is, e.g. "/usr/lib/jvm/jre-1.6.0-openjdk/bin")? If it doesn't set it accordingly ir /etc/profile, too.

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